1. TECHNICAL FIELD
The present invention relates to a semiconductor laser with a tunable gain spectrum.
The invention is usable as a frequency-tunable laser source or as an optical gain switching device and has applications in various fields such as e.g. optical telecommunications, lidars and spectroscopy.
The word xe2x80x9claserxe2x80x9d is used here in the broadest sense as a light amplifier with or without a cavity.
2. PRIOR ART
An article published in Appl. Phys. lett. 70 (10) on March 1997 written by N. K. Dutta, et al. discloses temperature variation-tunable laser diodes. Such diodes have a response time of approximately 1 second.
An article published in Electronic Letters, Jul. 17, 1986, Vol. 22, No. 15 by F. Favre, et al. discloses laser diodes tunable by means of an external diffraction grating. Such diodes have a response time of approximately 1 millisecond and are also ponderous and very costly.
An article published in Optical and Quantum Electronics 22 (1990) 1-15 by S. Murata, et al. discloses tunable laser diodes having an active region, a variable refractive index region and a region forming a distributed Bragg reflector, said regions being monolithically integrated on a substrate. Such diodes have a response time of approximately 1 nanosecond, but are difficult to manufacture and therefore expensive, whilst also requiring complex electronic means for controlling the wavelength of the laser radiation emitted, because three electric current are necessary for this purpose (namely one current for each of the aforementioned regions).
An article published in Applied Physics Letters 60 (24), Jun. 15, 1992 written by L. Y. Liu, et at. discloses the possibility of modulating the gain sspectrum during a laser emission by the quantum confined Stark effect. In the laser device described in this document, a quantum well active zone is formed in the intrinsic zone in a reverse biased P-I-N junction and the gain in the quantum well is obtained optically. Therefore such a laser device is not convenient.
The present invention aims at obviating the aforementioned disadvantages by proposing a wavelength-tunable laser diode with edge emission, which is easier to manufacture and therefore less expensive than the laser diodes known from the Murata article (it requires only a single control current), whilst being fast, its response time, i.e. its wavelength switching time, being approximately 1 nanosecond or less.
The tuning principle of this laser diode is based on a modulation by an electrooptical effect such as the quantum confined Stark effect of the gain spectrum of the diode during the emission of the laser radiation. The active zone of said laser diode is appropriately designed to ensure that the single injected current simultaneously produces a gain in the active layer and a space charge field permitting the modulation of the wavelength of the laser radiation.
In all the preferred embodiments of the invention, said wavelength is directly controlled by the current injected into the laser diode. However, in certain special embodiments, said wavelength can be controlled by an auxiliary laser beam.
In other special embodiments, the laser according to the invention can operate entirely optically. In this case, either the gain and the wavelength modulation are produced by a pumping laser beam, or the gain is produced by a pumping laser beam and the wavelength modulation by an auxiliary laser beam.
More generally, the invention proposes a tunable laser, whose wavelength can be very rapidly modified with a very short switching time of approximately 1 ns or less.
More specifically, the present invention relates to a tunable laser comprising a semiconductor heterostructure, said laser being characterized in that the heterostructure comprises an active zone having at least one quantum well called an active quantum well and which emits a laser radiation during the introduction of charge carriers into the active zone and at least one other quantum well called the collection quantum well, on either side of the active quantum well, the collection quantum wells being provided for collecting and confining part of the charge carriers introduced, and means for distributing charge carriers in the collection quantum wells so as to create a space charge field for acting on the active quantum well during the emission of the laser radiation by modifying, by an electrooptical effect, the gain spectrum of said active quantum well.
Thus, a variation in the number of charge carriers introduced is able to modify said space charge field and therefore the wavelength of the laser radiation.
Preferably, the lowest optical transition of each of the collection quantum wells is at an energy higher than that of the active quantum well.
Preferably, the fundamental states of the valence and conduction bands of the heterostructure are in the active quantum well.
According to a first special embodiment of the laser according to the invention, said laser forms a laser diode, the heterostructure forming a P-I-N junction which is to be forward biased and which has an intrinsic zone between a type P zone and a type N zone, the active zone being formed in said intrinsic zone, the laser radiation being emitted during the injection of a charge carrier flow into the active zone.
In this case, the charge carrier distribution means can comprise means for the forward biasing of the P-I-N junction, the inhomogeneity of the injection current in the active zone being responsible for the separation of the charge carriers.
According to a second special embodiment, the charge carrier distribution means comprise two piezoelectric barriers, which are respectively placed on either side of the active quantum well and which, by means of their piezoelectric fields, give rise to an accumulation of electrons and holes on either side of the active quantum well.
In said second embodiment, the active quantum well may not have a piezoelectric field or may have a piezoelectric field in the opposite direction to that created by the piezoelectric barriers or can have a piezoelectric field in the same direction as that created by the piezoelectric barriers.
Moreover, in said second embodiment, the collection quantum wells can be designed so that the times taken by the charge carriers for escaping said collection quantum wells are approximately 1 nanosecond.
According to a third special embodiment of the laser diode according to the invention, the active zone comprises at least two piezoelectric barriers and at least two collection quantum wells on either side of the active quantum well, the charge carrier distribution means comprising said piezoelectric barriers.
In this case, the forbidden band width of each of the collection quantum wells, which are respectively the most remote from the active quantum well, preferably exceeds those of each of the other collection quantum wells, so that the carriers can pass through by a tunnel effect.
In the case of the second and third embodiments, the number of charge carriers introduced can be kept constant, the wavelength of the laser radiation then being controlled by an auxiliary laser beam, whose photons have an energy greater than the forbidden band width of each piezoelectric barrier and whose intensity is sufficiently high to optically induce a charge of the heterostructure.
In this case, the charge carriers can be introduced by means of a laser beam for pumping the active zone. This laser beam for pumping the active zone can also constitute the auxiliary laser beam (also in order to control the wavelength).
As has been shown hereinbefore, in certain special embodiments of the invention, the separation of the charge carriers on either side of the active quantum well results from the introduction of piezoelectric layers into the active zone of the laser. For crystalline semiconductors of the zinc blende type, said piezoelectric layers are preferably stressed layers grown by epitaxy on the surface of the heterostructure substrate, the latter having high crystallographic indices (n, 1, m) with nxc3x971xc3x97mxe2x89xa00, such as e.g. a substrate (111) or (211).
In this connection reference can e.g. be made to a published article by E. Anastassakis in Physical Review B, Volume 46, Number 8, Aug. 15, 1992, which shows the biaxial stress present in such layers produces therein a strong piezoelectric field along the growth axis.
In such semiconductors of the zinc blende type, the piezoelectric fields can easily reach values exceeding 100 kV/cm for relatively moderate mixmatching stresses of the crystal lattices of approximately xcex5=0.7% and in this connection reference can be made to a published article by R. Andre, et al. in Physical Review B, Volume 42, Number 17, Dec. 15, 1990.
The electrooptical effect used in the present invention is preferably the quantum confined Stark effect. However, other effects, such as e.g. the Franz-Keldysh or Wannier-Stark effect can also be used.